Pipeline Circumferential Cracking in Near-Neutral pH Environment Under the Influence of Residual Stress: Crack Growth

Circumferential near-neutral pH corrosion fatigue (C-NNpH-CF) is the result of the simultaneous impact of axial residual and applied stresses along with the near-neutral pH corrosive environment established on the external surface of the buried pipeline because of leakage through the protective coating. (This mechanism has previously been referred to as near-neutral pH stress corrosion cracking.) Since integrity management measures should be implemented before Stage III (rapid crack propagation to rupture), this study aims to evaluate the effect of bending residual stress (a suitable source of axial residual stress) and cyclic loading (simulated pipeline pressure fluctuations) on crack growth at Stage II. Based on the digital image correlation (DIC) method, the final stress distribution in length and thickness direction was used to analyze crack growth in various test parameters, including applied cyclic loading, initial notch depth/position, and bending angle/direction. As a result of stress gradients in the depth direction of bent pipelines, a new method was developed to obtain the stress intensity factor. A comparison of crack growth rates between circumferentially oriented and longitudinally oriented NNpH-CF was performed to reveal the growth mechanism. Crack growth was maximum at 1 mm depth initial notch, 20 deg bend (inward), and 50 pct cycling load.